A Raman laser generally speaking comprises a pumping laser source which emits a pump source frequency, and a cavity containing a Raman medium. The laser pump source promotes a Stokes field in the cavity, thereby eliciting Raman scattering. Raman scattering occurs when an incident photon interacts with a molecule (or an atom) and generates a red-shifted photon with the change in wavelength resulting from the conservation of energy when the excitation in the molecule occurs. The energy E of a photon can be described as the product of its frequency .nu. and Planck's constant h (i.e. E=.nu.h, where h=6.63.times.10.sup.-34 J-s).
It is readily apparent that when the incident photon having an energy E, interacts with a molecule (or atom) there is a transfer of energy. The contacted molecule (or atom) accepts the charge in energy, +.DELTA.E, which can be a form of molecular (or atomic) rotation/vibration, or electronic rotation/vibration. The photon, consequently, loses the change in energy, -.DELTA.E. This loss of energy, -.DELTA.E, is translated into a change in the associated frequency -.DELTA..nu. of the photon. Because the frequency .nu. is inversely proportional to the wavelength .lambda. of the photon, it is clear that with a decrease in photon frequency .nu., there is a corresponding increase in wavelength .lambda..
Because the scattered photon is less energetic than the pump photon, the scattered photon has a longer wavelength. Having a resonant cavity containing the Raman medium therefore creates a Raman laser output beam. A desired Raman laser would be driven by a low powered pump source beam, be a continuous wave (cw) laser, and be widely tunable (attaining off-resonant scattering).
One type of prior art Raman laser includes those driven by a high-power pulse laser pump source. At high laser intensities the Raman process can have gain and produce stimulated off-resonant Raman scattering, and, because of the high intensities needed, off-resonant stimulated Raman scattering is most often studied in the high-power, pulsed regime. While these systems provide off-resonant Raman pulsed scattering, they are not provided in a continuous wave regime, and they require large amounts of power.
A second type of prior art Raman laser includes near-resonant Raman lasers driven by a continuous wave laser. Raman lasers use this inelastic scattering process to create Stokes-shifted outputs. However, because of the lower intensity of the cw pump lasers, these cw Raman lasers generally operate near a molecular (or atomic) resonance to increase the Raman gain and therefore are tunable only over narrow regions near the resonance. A few examples include a 67 .mu.m cw Raman laser in NH.sub.3, a cw Na Raman laser near the D lines, a two-photon-pumped cw Rb Raman laser near 776 nm, and various cw Raman lasers near Ne resonance in a He--Ne laser discharge tube. Although cw Raman lasers have been built to operate near a molecular or atomic resonance where the Raman gain is large, these lasers are tunable only over a narrow region near the molecular or atomic resonance. Furthermore, the pump wavelength needed for these cw Raman lasers is dictated by the energy-level structure of the molecule or atom. Consequently, while these systems provide Raman scattering in a continuous wave (cw) regime at a lower intensity to that compared to the pump actuated Raman lasers, they are tunable only over narrow regions near the resonance.
Finally, a third type of Raman laser includes those composed of optical fibers. Continuous wave Raman lasing is also possible in optical fibers in which the long interaction length of the fiber and the small spot size in the fiber increase the gain so Raman lasing can occur. However, input pump powers of .about.1 W are typically needed to pump these Raman fiber lasers. In addition, the Raman shift in a fiber is only .about.440 cm.sup.-1, which is inconvenient if one is looking for substantial shifts of wavelength. While these systems do provide Raman scattering in a continuous wave (cw) regime, such lasers are driven at a moderate power level and additionally are tunable only over narrow regions near the resonance.
While the above prior art systems provide important advantages over one another, none of the systems provide all three desired features of a Raman laser as described previously, namely a low power pump laser source, a continuous wave, and a highly tunable or off-resonance laser beam. Accordingly, there remains a need for obtaining a highly tunable, continuous wave (cw) Raman laser with a low powered pumping source.